There are two buffers, IBUF (insert buffer) and FBUF (find
buffer). They are used as a typing target for the actions
described below. They both have a peculiar logic: when typing
is expected to fill a buffer, an empty value means "reuse
previous value". For example, typing "I foo I " inserts
"foofoo".

T ( n -- ): select line n for editing.
P xxx: put typed IBUF on selected line.
U xxx: insert typed IBUF on selected line.
F xxx: find typed FBUF in block, starting from current
       position+1. If not found, don't move.
i xxx: insert typed IBUF at cursor. "i" is to avoid shadowing 
       core word "I".   
Y: Copy n characters after cursor into IBUF, n being length of
   FBUF.                                                (cont.)